signpsbt: add utxo info to inputs

If you build a PSBT externally from CLN and attempt to sign for the
output, we would crash. Now we don't crash.

Changelog-Changed: JSON-RPC: `signpsbt` will now add redeemscript + witness-utxo to the PSBT for an input that we can sign for, before signing it.

Fixes ElementsProject#5499 ?

tests/test_wallet.py

@@ -695,6 +695,35 @@ def test_utxopsbt(node_factory, bitcoind, chainparams):
                     reservedok=True)
 
 
+def test_sign_external_psbt(node_factory, bitcoind, chainparams):
+    """
+    A PSBT w/ one of our inputs should be signable (we can fill
+    in the required UTXO data).
+    """
+    l1 = node_factory.get_node(feerates=(7500, 7500, 7500, 7500))
+    amount = 1000000
+    total_outs = 4
+
+    # Add a medley of funds to withdraw later, bech32 + p2sh-p2wpkh
+    for i in range(total_outs // 2):
+        bitcoind.rpc.sendtoaddress(l1.rpc.newaddr()['bech32'],
+                                   amount / 10**8)
+        bitcoind.rpc.sendtoaddress(l1.rpc.newaddr('p2sh-segwit')['p2sh-segwit'], amount / 10**8)
+
+    bitcoind.generate_block(1)
+    wait_for(lambda: len(l1.rpc.listfunds()['outputs']) == total_outs)
+
+    # Build a PSBT using all our inputs, externally
+    inputs = []
+    for inp in l1.rpc.listfunds()['outputs']:
+        inputs.append({'txid': inp['txid'], 'vout': inp['output']})
+    addr = l1.rpc.newaddr()['bech32']
+    psbt = bitcoind.rpc.createpsbt(inputs, [{addr: (amount * 3) / 10**8}])
+
+    l1.rpc.reserveinputs(psbt)
+    l1.rpc.signpsbt(psbt)
+
+
 def test_sign_and_send_psbt(node_factory, bitcoind, chainparams):
     """
     Tests for the sign + send psbt RPCs

wallet/walletrpc.c

@@ -648,6 +648,33 @@ static struct command_result *match_psbt_inputs_to_utxos(struct command *cmd,
 					    "Aborting PSBT signing. UTXO %s is not reserved",
 					    type_to_string(tmpctx, struct bitcoin_outpoint,
 							   &utxo->outpoint));
+
+		/* If the psbt doesn't have the UTXO info yet, add it.
+		 * We only add the witness_utxo for this */
+		if (!psbt->inputs[i].utxo && !psbt->inputs[i].witness_utxo) {
+			u8 *scriptPubKey;
+
+			if (utxo->is_p2sh) {
+				struct pubkey key;
+				u8 *redeemscript;
+				int wally_err;
+
+				bip32_pubkey(cmd->ld->wallet->bip32_base, &key,
+					     utxo->keyindex);
+				redeemscript = bitcoin_redeem_p2sh_p2wpkh(tmpctx, &key);
+				scriptPubKey = scriptpubkey_p2sh(tmpctx, redeemscript);
+
+				tal_wally_start();
+				wally_err = wally_psbt_input_set_redeem_script(&psbt->inputs[i],
+									       redeemscript,
+									       tal_bytelen(redeemscript));
+				assert(wally_err == WALLY_OK);
+				tal_wally_end(psbt);
+			} else
+				scriptPubKey = utxo->scriptPubkey;
+
+			psbt_input_set_wit_utxo(psbt, i, scriptPubKey, utxo->amount);
+		}
 		tal_arr_expand(utxos, utxo);
 	}